Flexible mandrels have long been used to support the walls of metal tubing as it is bent. Such mandrels generally consist of a rigid mandrel body and a series of ball or ring members, each of which is locked on to a rigid inner ball link. The links are flexibly interconnected so that the string of balls may flex during the bending or forming process. Tube bending mandrels of this type are disclosed, for example, in U.S. Pat. No. 3,190,106, issued June 22, 1965, to H. M. Spates and have performed well in bending tubing of many different sizes of materials. The routine availability of mandrels for a wide variety of tube bending operations with ordinary tubing materials has naturally stimulated interest in extending the limits of these techniques to include the production of so-called "difficult" bends, e.g., those involving tight radii, compound bends, and bends in large diameter or thin walled tubes of the more exotic high strength metals and alloys. Requirements for bends of a difficult nature presently originate most often in the aero-space industry, but many other uses of such tubing may develop when the technology of producing the difficult bends becomes reliable.
Among the problems encountered in the use of conventional and prior art mandrels to produce tight bends or bends in thin walled exotic metal tubing is the high rate of wear and breakage of the mandrel links due to the extremely large and concentrated mechanical stresses imposed on them during the bending operation. Attempts to alleviate these problems by enlarging the parts for greater strength conflict with the need to keep the linkages small in order to provide close-spaced support for the walls of the tubes being bent, and to permit the mandrels to be flexed into tight radius curves. One type of prior art device used to form difficult bends in ordinary or "soft" metal tubing is the cable mandrel, in which the flexible chain of rigid ball links is replaced by a high strength flexible steel cable held taut between the mandrel body and a special terminal link at the free end of the mandrel. A close pitch or spacing of the balls in the cable mandrel is achieved by using a so-called "reverse ball" configuration in which each ball is designed to fit or nest inside the preceding ball as the string of balls is flexed, instead of riding on the outer surface of the preceding ball as in the more conventional rigid link mandrel.
While cable mandrels have been used to produce bends not otherwise feasible, they suffer a number of drawbacks which prevent their routine application in high volume production situations. Most of these limitations are directly attributable to the characteristics of the wire cables used to link the mandrel balls. For instance, the elastic characteristics of a wire cable are determined by the properties of the filaments and strands of which it is constructed, and these properties exhibit a statistical dispersion. As a result, a considerable portion of the cable's potential strength is unused under most conditions, while the load is very high on those elements which normally carry it. It is not surprising, therefore, that cables tend to stretch and to break, requiring periodic adjustment and replacement. Moreover, cables have relatively low transverse strength and are subject to shearing over the inner surfaces of the ball members which they contact while under the heaviest loads during tube bending operations. The more or less continuous need for time consuming adjustment and replacement of cables makes these mandrels costly to use and has substantially limited their application. And, of course, the limitations of the cables are most dramatic when the tubes to be bent or the bends to be performed are in the "difficult" category.